1. Field of the Invention
The present invention relates to an optical semiconductor device and, more particularly, to an optical semiconductor device that carries high-frequency signals used for optical communication or the like.
2. Description of the Related Art
In recent years, with the advancing broadband optical communication, the demand for transmitting a larger volume of information at lower cost has been accelerating concurrently with the increasingly disseminating public communication networks that use optical fibers. To increase the volume of information at low cost, it is necessary to achieve higher transmission speed. The transmission has been increased from 600 Mbps to 2.5 Gbps, and it is gradually being further increased to 10 Gbps. With this trend, light emitting devices and light receiving devices used with optical transmitters and receivers are being required to exhibit stable performance at high speed. At the same time, it is essential to develop inexpensive, highly efficient optical devices.
Many optical devices use inexpensive can packages shaped like trunk shafts to maintain low cost. The body of a can package is called a stem, and the stem is provided with a few rod-shaped lead terminals to propagate electrical signals. In an optical device using the can package, the lead terminals and a circuit board having a drive IC for driving the optical device are connected by soldering.
However, the lead terminals of the can package have high impedance, so that when the optical device is connected to the circuit board, electrical signals are reflected in their connection area, resulting in deteriorated signal waveforms as the frequency of an electrical signals increases. In particular, longer lead terminals make the deterioration of signal waveforms more prominent. It is, therefore, necessary to shorten the lead terminals to be connected to the circuit board as much as possible.
On the other hand, if the lead terminals are too short, then an external force applied to a can package cannot be absorbed by flexure of lead wires, resulting in a high stress to be developed at a soldered portion of the circuit board. This causes damage, such as unsoldering, in some cases.
As a solution to the aforesaid problem, a method has been used, in which the lead wires of the can package are soldered to a flexible substrate having transmission lines provided on a flexible strip-shaped dielectric film, and the can package is connected to the circuit board through the intermediary of the flexible substrate. This mounting method allows external forces applied to the can package to be absorbed by the flexure of the flexible substrate.
As a publicly known example of an optical device using such a flexible substrate, a configuration in which a laser package is mounted on a flexible substrate has been disclosed in, for example, Honeywell Application Note (HVAN 1 Rev 2; 0603, Honeywell VCSEL Optical Products, “Designing with the Honeywell 10 Gbps TOSA and ROSA”, page 1 of 29 to page 4 of 29).
If static electricity or a surge voltage is applied to a lead terminal, a laser element provides an output beyond a maximum permissible output level, deteriorating the laser element. To restrain the deterioration of the laser element, a construction, in which a capacitor is provided in parallel to a semiconductor laser element and a resistor is provided in series with the semiconductor laser element in a can package, has been disclosed in, for example, Japanese Patent Laid-Open No. 2001-320125 (refer to the 1st line to the 4th line in the top right column of page 2, and the 5th line to the 10th line in the bottom left column of page 2; and also see FIG. 1 to FIG. 4).
However, if an optical device having the flexible substrate connected to the lead terminals of the can package by using the flexible substrate described above is connected to the circuit board for driving the optical device, reflection of high frequencies attributable to impedance mismatch takes place to a certain degree at a connection area between the circuit board and the flexible substrate and a connection area between the flexible substrate and the lead terminals of the can package.
A flexible substrate usually has a length of about 10 mm, so that if the reflected high frequencies described above are present, the flexible substrate acts as if it were a resonator having its both ends serving as base points, causing interference of high frequencies to take place. This generates surges having pitches of a few GHz when attention is focused on the frequency characteristics of transmission characteristic S21 and reflection characteristic S11. The surges cause a problem in that the waveforms of high-frequency signals propagating on the flexible substrate are distorted.
Taking a semiconductor laser device, for example, reflection S11 of high frequencies that takes place between the flexible substrate and the can package is normally larger than the reflection that takes place between the flexible substrate and the circuit board. This is because the impedance of the whole can package, including the resistance of a laser diode constituting the can package, the capacity of the stem, and the inductance of the lead terminals, changes intricately according to frequencies.
Hence, in a semiconductor laser device that performs direct modulation, high-frequency modulation signals that propagate on the flexible substrate are influenced by the surges of transmission characteristic S21 or reflection characteristic S11, posing a problem in that the modulation waveforms of the semiconductor laser are distorted, thus making it impossible to obtain good high frequency characteristics.